This application is related to Japanese Patent Application No. Hei 11(1999)-355274 filed on Dec. 15, 1999, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a light modulation type liquid crystal display device having a high brightness and a high contrast ratio, and more particularly, a reflection-type direct-viewing liquid crystal display, a transmission-type direct-viewing liquid crystal display, and a projection-type liquid crystal projector.
2. Description of the Related Art
A display systems of liquid crystal display devices, a TN (Twisted Nematic) mode and a STN (Super-twisted Nematic) mode which make use of birefringence and optical rotation of a liquid crystal are generally adopted currently. Since these systems make use of such birefringence and optical rotation of the liquid crystal, it is necessary to use two sheets of polarizers and hence, light absorption loss amounts to approximately 60%, which darkens a display. On the other hand, another system which does not use the polarizers has been proposed. It is a guest-host (G-H) system which is a system in which a dichroic dye is added to a liquid crystal, or a system which makes use of scattering of light as represented by a dynamic scanning mode, a phase change mode and a polymer dispersion mode. Since the later system does not suffer from the loss of light caused by the polarizers, it enable a brighter display compared with the former system.
In case p-type dichroic dye is used in the above-mentioned guest-host system, the light absorption of the dichroic dye becomes maximum when the polarizing direction of light is parallel to a long axis of molecules, and becomes minimum when an optical path of the light and the long axis of molecules are arranged parallel to each other (in case an n-type dichroic dye is used, an opposite phenomenon occurs). Accordingly, a liquid crystal display element adopting a popular homogeneous orientation can obtain a two-value state. That is, when an electric field is not applied, a colored display is obtained by horizontal alignment of the liquid crystal molecules and the dichroic dye, while when the electric field is applied, a colorless display is obtained by vertical alignment of the liquid crystal molecules and the dichroic dye.
Here, to increase the brightness and the contrast without using polarizers, there has been known a method which uses a guest-host liquid crystal layer having a multi-layered structure, and a method which makes use of a phase change mode of a cholesteric liquid crystal. However, the fabrication of a panel having the multi-layered structure is difficult. Further, this method gives rise to other problems such as an expensive fabrication cost and a double reflection of images. Further, the phase change mode of the cholesteric liquid crystal is inferior to the TN mode in basic characteristics such as contrast, drive voltage, response speed and the like. Therefore, these methods have not been put into practice.
On the other hand, the display system which makes use of the scattering of light, in general, includes a reflection-type liquid crystal display device having a black light absorption body in the back. Due to such a constitution, when a liquid crystal layer is in the transparent state, a black display is realized, while when the liquid crystal is in the scattered state, a white display is realized due to the backward scattering of light. Since the dynamic scattering mode or the phase change mode of the above system is not practical in terms of reliability, response speed, drive voltage and the like, a polymer dispersion mode has been currently extensively studied. For example, Japanese Patent 2,724,596 and Japanese Patent re-publication through PCT 501,631/1983 disclose a polymer dispersion type liquid crystal. In these publications, depending on the presence or non-presence of an electric field, a refractive index of the liquid crystal vis-à-vis the polymer is changed so as to change the liquid crystal between the scattered state and the transparent state thus enabling an image display.
However, in general, the refractive index of an organic material such as a liquid crystal or polymer is limited to within a narrow range, and its ratio is approximately 1.2 at maximum; hence, the back scattering ratio of the light scattering type liquid crystal display element is low, thereby making it difficult to obtain sufficient brightness. Accordingly, it becomes necessary to increase the thickness of the cell to obtain a large back scattering ratio. In this case, however, the drive voltage becomes extremely high and hence, the device becomes unrealistic.
In view of the above, a system in which a lower electrode of a scattering type liquid crystal layer has a mirror surface which is provided with a smooth rugged structure having a specific inclination angle is disclosed in Japanese Laid-Open Patent Publication 114,797/1996. Here, by effectively using incident light, the intensity of light reflected in the direction normal to the surface of the substrate can be increased without giving a scattering function to the reflection plate. However, this system is insufficient in the black display, and further, brightness at the white display is so insufficient that a paper white display cannot be obtained.
Besides the above-mentioned methods, methods which combine the scatter-type liquid crystal with various optical elements have been proposed by several publications, including the following publications. Japanese Laid-Open Patent Publication 317,702/1994 proposes a method which uses an inclined reflection plate and an inclined light absorption body. Japanese Laid-Open Patent Publication 87,006/1996 proposes a method in which light scattering means such as a prism is provided between a scatter-type liquid crystal layer and a colored plate. Japanese Laid-Open Patent Publication 62778/1998 proposes a method in which a low refractive index layer is placed on a back reflection plate. However, all of these methods have a deterioration in visual angle characteristics and are poor in brightness.
The present invention has been made in view of the above problems and it is an object of the present invention to provide a liquid crystal display device which is capable of obtaining a high brightness and a high contrast ratio without a polarizer or with a single polarizer.
To achieve the above-mentioned object, a reflection-type liquid crystal display device according to the present invention includes upper and lower substrates which respectively have upper and lower electrodes, the upper and lower substrates being arranged in parallel with the upper electrode facing the lower electrode; and a liquid crystal layer sandwiched between the upper and lower substrates, the upper substrate being transparent and having protruding portions adjacent to the liquid crystal layer, each protruding portion having a surface downwardly inclined to defined a prism, the lower substrate having light reflection portions and light absorption portions, wherein when an electric field is applied to the liquid crystal layer, incident light into the upper substrate is selectively directed to the light reflection portions and the light absorption portions by means of the protruding portions and the liquid crystal layer.
Due to such a constitution, in displaying a bright state, the incident light from the outside is directed to be incident on the light reflection portions so that the light is reflected to a viewer side and thereby a sufficient brightness can be obtained. On the other hand, in displaying a dark state, the incident light is directed to be incident on the light absorption portions so that the light is not reflected to the viewer side and thereby a sufficiently high contrast ratio can be obtained.
Further, a transmission-type liquid crystal display device according to the present invention includes upper and lower substrates which respectively have upper and lower electrodes, the upper and lower substrates being arranged in parallel with the upper electrode facing the lower electrode; and a liquid crystal layer sandwiched between the upper and lower substrates, the upper substrate being transparent and having protruding portions adjacent to the liquid crystal layer, each protruding portion having a surface downwardly inclined to defined a prism, the lower substrate having light reflection portions and light absorption portions, wherein when an electric field is applied to the liquid crystal layer, incident light into the upper substrate is selectively directed to the light transmission portions and the light absorption portions by means of the protruding portions and the liquid crystal layer.
Due to such a constitution, in displaying a bright state, incident light from the outside to the device is directed to be incident on the light transmission portions so that the light is transmitted to a viewer side and thereby a sufficient brightness can be obtained. On the other hand, in displaying a dark state, the incident light is directed to be incident on the light absorption portions so that the light does not reach the viewer side and thereby a sufficiently high contrast ratio can be obtained.